Cell therapy for the treatment of liver disease is a promising alternative to organ transplantation. The feasibility of cell transplantation to correct enzyme deficiency and provide a bridge to liver transplantation has been demonstrated in human trials, but this approach is limited by a severe shortage of liver donors and hepatocytes. Therefore, a long-term aim in the field is to identify alternative sources of hepatocytes that coul potentially be useful for cell therapy. One potential source is the intrahepatic progenitor/stem cells that normally differentiate into both bile ducts and hepatocytes throughout adult life. Recently it became possible to prospectively isolate liver progenitor cells from adult mice and humans, to establish self-renewing cultures with these cells in vitro, and to perform genetic lineage tracing in vivo. In this application, we propose to utilize new methodology to isolate human liver progenitor cells and expand them in culture, and to test the ability of these human cells to repopulate the liver in a cell-transplantation model (Aim 1). Second, we propose to understand the requirement for these cells in a model of progressive liver injury (Aim 2). This research is relevant to understanding the mechanisms of liver repair in chronic injury and how these stem cells might be used for transplantation to treat liver diseases.